blockchain: Correct invalid assertion.

This corrects the assertion in the decodeSpentTxOut function so it does
not improperly cause a panic when unwinding transactions during a reorg
under certain circumstances.  In particular, the provided transaction
version that is passed when a stxo entry does not exist is now -1 in
order to properly distinguish it from the zero value.

It also updates the tests accordingly.

This was discovered by the reorg on testnet from block
00000000000018c58c2d2816f03dac327d975a18af6edf1a369df67ecddaf816 to
0000000000001c1161a367156465cc6226e9f862d9c585f94db5779fdf5455ff.

blockchain/chainio.go

@@ -298,7 +298,7 @@ func decodeSpentTxOut(serialized []byte, stxo *spentTxOut, txVersion int32) (int
 		// it.  This should never happen unless there is database
 		// corruption or this function is being called without the
 		// proper state.
-		if txVersion == 0 {
+		if txVersion == -1 {
 			return offset, AssertError("decodeSpentTxOut called " +
 				"without a containing tx version when the " +
 				"serialized stxo that does not encode the " +
@@ -382,7 +382,7 @@ func deserializeSpendJournalEntry(serialized []byte, txns []*wire.MsgTx, view *U
 			// to detect this case and pull the tx version from the
 			// entry that contains the version information as just
 			// described.
-			var txVersion int32
+			txVersion := int32(-1)
 			originHash := &txIn.PreviousOutPoint.Hash
 			entry := view.LookupEntry(originHash)
 			if entry != nil {

blockchain/chainio_test.go

@@ -194,8 +194,9 @@ func TestStxoDecodeErrors(t *testing.T) {
 			bytesRead:  2,
 		},
 		{
-			name:       "no serialized tx version and passed 0",
+			name:       "no serialized tx version and passed -1",
 			stxo:       spentTxOut{},
+			txVersion:  -1,
 			serialized: hexToBytes("003205"),
 			errType:    AssertError(""),
 			bytesRead:  1,